205/70-15 is probably the best choice if you want to stick with the stock 5.5" wheel width or a 6" wheel width. Using the non-Java script miata.net tire calculator (
https://www.miata.net/garage/tirecalc.html ) change the stock size to 185/80-15 and then use 195/70-15, 205/70-15, 215/65-15 in the tires to compare box.
It will come back with the 185/80-15 showing 757 revs/mile, 195/70 showing 783 revs/mile for a -3.4% difference, the 205/70 showing 767 revs/mile for a -1.3% difference and the 215/65 showing 776 revs/mile for a -2.4% difference.
While the 195 and the 205 section width tires seem reasonably happy on a 5.5" wide wheel, the 215 section width needs a 6" wheel width as a minimum. Due to the difference in backspacing, even at the same offset, the wider wheel and tire will stick outboard a bit more than the stock wheel and I have seen some front fender clearance issues associated with the 215/65-15 and 6" or wider wheels on a TR6.